The present invention relates generally to an improved injection molding machine and method of using the machine to form net shape molded parts. More specifically, the present invention relates to a plunger molding machine for use in molding reinforced polymer compositions, particularly, polymers loaded with thermally conductive filler media, such as carbon, ceramics and metallic material in the form of fibers and flakes.
In the molding industry, it has been well known to injection mold plastic materials into various articles of commerce. In particular, it has become well known to load such plastics or polymer-based compositions with filler materials to form a reinforced polymer composition. Reinforcing a polymer composition with other media is done for many different purposes. For example, a reinforced polymer composition may be employed to provide a thermally conductive plastic where the reinforcing material is highly thermally conductive, such as is the case with carbon fiber or aluminum flakes. Another example includes an application where the polymer is loaded with copper fiber to provide an electrically conductive polymer composition. Still further, aluminum flakes may be loaded in the polymer composition to provide a composition that includes EMI shielding properties. Also, glass, carbon or other structural fibers may be employed to add strength and/or stiffness.
In general, the loading of a polymer base matrix, with a reinforcing material, raises many concerns regarding the ability to successfully injection mold such a composition due to the presence of the additional suspended reinforcing material. For example, if the reinforcing material that is loaded into the polymer matrix is long carbon fiber, there is a greatly increased potential for strand and/or filament breakage during the melting and molding process. During the molding process, the competing issues of thorough mixing of the loaded polymer composition and the concern of excessive breakage of the delicate reinforcing media must be balanced to achieve the desired product. Prior art molding machines typically create high turbulence and/or grinding of the polymer material for the purposes of mixing the composition. These prior art machines commonly included a torpedo-shaped member or spreader located in the center of the injection molding bore to increase the level of turbulence as the composition passes through the bore to cause the polymer to melt in a uniform manner and to improve the mixing of the composition. However, such turbulence and grinding of the polymer composition under pressure during the molding process results in increased reinforcing fiber breakage and greatly reduced reinforcement media length.
As a result, it can be clearly seen that these known molding processes are incompatible with the molding of thermally conductive polymer compositions as described above. In particular, a thermally conductive composition that employs carbon fiber reinforcing requires that the breakage or damage to the reinforcing fibers be kept to a minimum to ensure that the desired properties of the resulting composition are maintained. In the above example, if the lengths of the carbon fibers loaded within the polymer composition are ground up into much shorted lengths, it is clear that the overall thermal conductivity of the composition will be degraded as a result.
In an attempt to address the problems with breakage of reinforcing fibers, compression molding has been attempted where there is a manual lay-up of material and the reinforcing media thereon. As can be understood, such manual assembly is expensive and is far too slow for mass production. Thus, compression molding is inadequate and impractical for molding reinforced material and suffers from economic and geometry-related limitations.
In addition to the problems associated with the reduction of the length of reinforcing media, the alignment of the reinforcing fibers within the composition is also a concern. In the examples above, a highly aligned and oriented loading of reinforcing material along the path of conductivity is preferred to obtain higher performance of the molded composition. For example, a highly oriented array of carbon fiber within a polymer base would yield higher thermal conductivities than a composition that included randomly oriented fibers, because the number of transitions from carbon to polymer to carbon within the composition would be greatly reduced. Further, packing densities are higher when the fibers or filaments are well-aligned. The foregoing alignment and breakage problems become even more important where the aspect ratio of the reinforcing media becomes larger and larger.
Tapered bore injection molding machines have also been used to overcome the above noted deficiencies. However, while tapered bore machines preserve reinforcing fiber length and alignment, since there is no transfer of heat to the center of the bore the polymer melts in an uneven fashion and requires extended melt time within the injection molding bore.
In view of the foregoing, there is a demand for an improved injection molding machine and method that is well suited for accommodating polymer compositions loaded with reinforcing media having aspect ratios greater than 1:1 while enhancing the melt uniformity of the composition. Further, there is a demand for a molding machine that is capable of greatly decreasing the amount of breakage of reinforcing media during the molding process while enhancing the speed at which the polymer reaches its molten state. There is also a demand for a molding machine and method of using the machine that can better align reinforcing media along the line of melt flow to provide a better oriented reinforced composition.